1. Field of the Invention
This invention relates generally to magnetic float polishing and, more specifically, to a methodology for conducting magnetic float polishing of workpieces made of magnetic materials such as steel balls for bearing applications.
2. Background
Magnetic float polishing (MPF), sometimes termed magnetic fluid grinding, is a "gentle" polishing technique based on the magneto-hydrodynamic behavior of a magnetic fluid that can float non-magnetic abrasive grains suspended in it. The magnetic fluid is generally a colloidal dispersion of extremely fine (100 to 150 .ANG.) subdomain ferro-magnetic particles, usually magnetite (Fe.sub.3 O.sub.4), in water or hydrocarbon based carrier fluids such as kerosene. The ferrofluids are made stable against particle agglomeration by the addition of surfactants.
When a magnetic fluid is placed in a magnetic field gradient, it is attracted towards the side having a higher magnetic field intensity. If a non-magnetic substance (e.g., abrasive grains in this case) is mixed in the magnetic fluid, it is discharged towards the side having a lower magnetic field intensity. When the field gradient is set in the gravitational direction, the non-magnetic material is made to float on the fluid surface by the action of a magnetic buoyancy levitational force. The process is considered highly effective for finish polishing because the levitational force is applied to the abrasive grains in a controlled manner. The forces applied by the abrasives to a workpiece set in the fluid are extremely small (about 1 N or less).
Though extremely effective at providing high-performance, polished surfaces, magnetic float polishing has been used to finish only non-magnetic materials, such as advanced ceramic balls for bearing applications, particularly alumina (Al.sub.2 O.sub.3), zirconia (Zr0.sub.2), silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4). This limitation arises from the nature of the magnetic float polishing technique, which is based on the magnetohydrodynamic behavior of the magnetic fluid. Heretofore, magnetic fluid polishing has not been used in the finishing of magnetic materials, such as steel balls for bearing applications, as magnetic induction upon the workpiece would adversely impact the dynamics of the magnetic float polishing system.
The traditional manufacture of rolling element bearings of hardened chrome steels involves the rounding of cylindrical slugs followed by heat treatment, rough grinding, and finishing. The cylindrical slugs are cut from steel wire of proper size for the desired size of finished balls. These slugs are then rounded while in a soft "as cut" condition, such as by rolling the slugs between two plates which are rotated at low speed relative to each other. The rounded slugs are then heat-treated to harden them whereupon they are ground and lapped to form finished ball bearings. Surface finish is generally achieved by lapping with a diamond paste in oil. Lapping involves working the surface of the balls in a grooved track formed between two working surfaces. As the balls roll along the surfaces of the grooves, a sliding movement of varying magnitude is set up which constitutes the lapping force. This lapping force, in combination with suitable lapping agents, causes gradual altering of the balls to a substantially geometrically spherical shape.
Steel ball bearings are used extensively in industry for a myriad of applications. With the advent of vacuum melting technology, it is now possible to use steels with higher hot hardness, such as M50 tool steels, for applications requiring higher operating temperatures. However, the need to improve performance, conserve energy, and reduce costs to stay competitive has resulted in an increased emphasis on higher efficiency, higher load bearing capability, higher temperature capability, higher precision and rigidity, lower friction and wear, and longer and reliable life. Some believe that rolling contact steel bearings have reached their maximum potential and for many demanding applications alternate materials must be used as bearing elements.
It is in this environment that the use of ceramics with higher hardness, lower density, higher chemical stability, higher modulus, lower friction, and higher wear resistance than steel has arisen, and, in connection with the manufacture of ceramic ball bearings, the use of magnetic float polishing. Until the advent of magnetic float polishing, ceramic balls were finished using low polishing speeds (a few hundred rpm) and diamond abrasive as a polishing medium. It takes a considerable time (some 12-15 weeks) to finish a batch of ceramic balls in this fashion, and the use of diamond abrasives at high loads often results in deep pits, scratches and microcracks on the ceramic ball surface. Magnetic float polishing was developed to allow for higher removal rates and shorter polishing cycles by using high polishing speeds with very low level controlled forces and abrasives not much harder than the workpiece. Notwithstanding the successful use of ceramics in ball bearing applications, there remain drawbacks, including a relatively high cost of manufacture, their inherent brittleness, and lack of reliability in performance.
It is an object of the present invention to provide a method for conducting magnetic float polishing on magnetic workpieces, such as a steel balls, so that the advantages of magnetic float polishing may be applied to broader technologies such as the manufacture of rolling contact steel bearings to increase the potential for the use of lower cost steel ball bearings in demanding applications.
It is a further object of the invention that the method be performed utilizing existing magnetic float polishing hardware.